[EASY] Move out fee calculation from "into_solution" (#2662)

# Description
Baseline solver. Moves out fee calculation from the `into_solution`
function.

This is done as a preparation for a follow up PR, where I will not
calculate the sell token price, but instead build `native_price_request`
to buy weth in an exact amount needed to cover the gas costs (instead of
using 1ETH as a default buy value). The output sell amount of
`native_price_request` will directly give us the amount of sell token
needed to cover the gas costs - fee.

Related to #2624, where
the `native_price_request` uses 1ETH as buy amount and fails. I don't
see any good reason to use 1ETH here as buy amount, as it is not needed.
It is perfectly fine to use, for example, 0.01ETH amount that actually
covers the gas cost and it's more accurate.

crates/solvers/Cargo.toml

@@ -17,6 +17,7 @@ axum = { workspace = true }
 bigdecimal = { version = "0.3", features = ["serde"] }
 chrono = { workspace = true, features = ["serde"], default-features = false }
 clap = { workspace = true, features = ["derive", "env"] }
+derive_more = { workspace = true }
 ethereum-types = { workspace = true }
 ethrpc = { path = "../ethrpc" }
 futures = { workspace = true }

crates/solvers/src/boundary/legacy.rs

@@ -437,7 +437,7 @@ fn to_domain_solution(
                     match order.solver_determines_fee() {
                         true => execution
                             .exec_fee_amount
-                            .map(solution::Fee::Surplus)
+                            .map(|fee| solution::Fee::Surplus(fee.into()))
                             .context("no surplus fee")?,
                         false => solution::Fee::Protocol,
                     },

crates/solvers/src/domain/eth/mod.rs

@@ -1,5 +1,4 @@
-use {crate::util::bytes::Bytes, web3::types::AccessList};
-
+use {crate::util::bytes::Bytes, derive_more::From, web3::types::AccessList};
 mod chain;
 
 pub use {
@@ -45,6 +44,10 @@ impl From<U256> for Ether {
     }
 }
 
+/// A token amount in wei, always representing the sell token of an order.
+#[derive(Clone, Copy, Debug, Default, Eq, Hash, Ord, PartialEq, PartialOrd, From)]
+pub struct SellTokenAmount(pub U256);
+
 /// Like [`Gas`] but can be negative to encode a gas discount.
 #[derive(Copy, Clone, Debug, Default, PartialEq, Eq)]
 pub struct SignedGas(i64);

crates/solvers/src/domain/solution.rs

@@ -126,11 +126,7 @@ pub struct Single {
 impl Single {
     /// Creates a full solution for a single order solution given gas and sell
     /// token prices.
-    pub fn into_solution(
-        self,
-        gas_price: auction::GasPrice,
-        sell_token: auction::Price,
-    ) -> Option<Solution> {
+    pub fn into_solution(self, fee: eth::SellTokenAmount) -> Option<Solution> {
         let Self {
             order,
             input,
@@ -144,7 +140,7 @@ impl Single {
         }
 
         let fee = if order.solver_determines_fee() {
-            Fee::Surplus(sell_token.ether_value(eth::Ether(gas.0.checked_mul(gas_price.0 .0)?))?)
+            Fee::Surplus(fee)
         } else {
             Fee::Protocol
         };
@@ -299,15 +295,15 @@ pub enum Fee {
     Protocol,
 
     /// An additional surplus fee that is charged by the solver.
-    Surplus(U256),
+    Surplus(eth::SellTokenAmount),
 }
 
 impl Fee {
     /// Returns the dynamic component for the fee.
     pub fn surplus(&self) -> Option<U256> {
         match self {
             Fee::Protocol => None,
-            Fee::Surplus(fee) => Some(*fee),
+            Fee::Surplus(fee) => Some(fee.0),
         }
     }
 }

crates/solvers/src/domain/solver/baseline.rs

@@ -189,6 +189,9 @@ impl Inner {
                 }
 
                 let gas = route.gas() + self.solution_gas_offset;
+                let fee = sell_token_price
+                    .ether_value(eth::Ether(gas.0.checked_mul(auction.gas_price.0 .0)?))?
+                    .into();
 
                 Some(
                     solution::Single {
@@ -198,7 +201,7 @@ impl Inner {
                         interactions,
                         gas,
                     }
-                    .into_solution(auction.gas_price, sell_token_price)?
+                    .into_solution(fee)?
                     .with_id(solution::Id(i as u64))
                     .with_buffers_internalizations(&auction.tokens),
                 )